A modular multilevel converter device is a type of multilevel converter in which several sub-modules (SM) are connected in series to constitute a converter arm.
Such MMC converter device may exhibit a high voltage output and a high capacity output of the multilevel converter, and control the output voltage with stepped outputs.
The MMC converter device has advantages that a structure thereof is simple compared to that of the general multilevel converter and thus is easy to implement, and extra sub-modules are used thereby extending the life.
FIG. 1 is a configuration diagram showing a general MMC converter device.
The general MMC converter device includes, for example, three legs 13a, 13b, and 13c and six converter arms 11a, 11b, 11c, 12a, 12b, and 12c, each arm including a plurality of sub-modules connected in series with each other.
The converter arm may be configured with three upper arms 11a, 11b, and 11c and three lower arms 12a, 12b, and 12c. In the figure, for example, four sub-modules SM 10 are illustrated for each arm, but the number of sub-modules may be varied.
The MMC converter device converts a DC-side input voltage Vdc input through a bus into an AC-side output voltage through the three legs 13a, 13b, and 13c and outputs the AC-side output voltage. This output voltage varies depending on on/off states of sub-modules SM 10 included in each arm.
That is, for example, when four sub-modules are included in each of the upper arms and the lower arms, the output voltage may be adjusted according to the number of the sub-modules 10 in which the on state is maintained. Herein, it is possible to control the on/off states of the switch of each of sub-modules 10 in each arm in order to regulate the output voltage.
FIG. 2 is a view showing an example of a sub-module of a general MMC converter device.
Referring to FIG. 2, each of the sub-modules (SM) 10 includes a half bridge circuit having a pair of semiconductor switches 21 and a capacitor 22 connected in parallel to these semiconductor switches 21. The semiconductor switch 21 includes a power semiconductor element 21a and a diode 21b connected in anti-parallel therewith.
The MMC converter device in the related art has many countermeasures against AC faults, but has no countermeasures against DC faults. That is, in the case of a sub-module configured with a half bridge circuit in the related art, there is a problem that the fault current cannot be cut off because the fault current flows from the AC side only to the diode 21b when a DC fault occurs. In order to block the fault current, a sub-module configured with a full bridge circuit in the related art has been proposed.
FIG. 3 is a view showing another example of a sub-module of a general MMC converter device.
Referring to FIG. 3, each of the sub-modules SM includes a full bridge circuit having two pairs of semiconductor switches 31 connected in parallel to each other, and a capacitor 33 connected in parallel to the semiconductor switches 31. The semiconductor switch 31 includes a power semiconductor element 31a and a diode 31b connected in anti-parallel therewith.
As described above, in the case of the sub-module configured with the full bridge circuit, there is an advantage that the fault current can be cut off because a reverse voltage is applied across the capacitor 33 due to the fault current from the AC side at the occurrence of a DC fault, but there is a disadvantage that a loss due to the switching operation of the switch 31 is increased. In fact, when configuring a full bridge circuit other than the half bridge circuit using the same number of sub modules, a switching loss of about 30% or more is caused.
In addition, in the case of the MMC converter device of FIGS. 2 and 3, when a fault occurs in the line while two MMC converter devices are connected to each other through a line to transmit/receive power to/from each other, there are problems that it is difficult to quickly cut off the fault current on the DC line and a loss is caused.